The present invention relates to the manufacturing of semiconductor devices, and more particularly, to reflow welding processes that generate flux residue.
Reflow furnaces are used for the reflowing of solder, also known as reflow welding, during the assembly of semiconductor devices by the surface mounting of electrical components to a circuit board or other substrate. With reference to FIG. 1, a typical reflow furnace 10 according to the prior art is illustrated. The furnace 10 includes a conveyor belt 12 for passing the electronic components into a number of different zones 14, 16, 18, 20, which are generally divided into a preheat zone 14, a soak zone 16, a reflow zone 18, and a cooling zone 20.
Before the circuit board enters the furnace 10, depending upon the type of component being reflow welded, either solder paste or flux paste/liquid is applied to the areas to be soldered. Solder paste comprises solder as well as flux and other additives, such as a solvent, whereas flux paste does not include solder. Flux paste can be used with, for example, flip chips because the flip chip die already includes solder bumps.
Solder is used to form a metallurgically sound solder joint, which will both hold the various electronic components in place and conduct electrical signals. Flux has a variety of purposes, which include removing oxides from metallization on the circuit board; removing oxides on the molten solder to reduce the surface tension and enhance flow; inhibiting subsequent oxidation of the clean metal surfaces during soldering; and assisting in the transfer of heat to the joint during soldering.
Upon entering the preheat zone 14, the initial heating of the circuit board begins. The process window for the preheat zone 14 is a 1xc2x0 to 3xc2x0 C./second rise in temperature to between 100xc2x0 C. to 125xc2x0 C. During this time, the solvent in the solder paste begins to evaporate. In the thermal soak zone 16, the circuit board is raised to 150xc2x0 C. to 175xc2x0 C. in about 60 to 120 seconds. This exposure allows the solder paste to dry and the flux to activate. In the reflow zone 18, the solder is heated to above its melting temperature thereby reflowing to form solder joints. The time during which a solder joint is molten is approximately 60 to 120 seconds, and the peak temperature of the leads in the solder joints is typically 220xc2x0 C. xc2x15xc2x0 C. Upon reaching the cooling zone 18, the circuit board is cooled at a rate not more than 4xc2x0 C./second, during which time the solder joints solidify. These profile times and temperatures can vary depending upon the type of semiconductor device, circuit board size, board density, throughput requirements, type of reflow equipment, and solder paste.
Many problems associated with this process are generated by use of the flux. Depending upon the type of flux paste used, a flux residue can remain after reflow welding. The residue can comprise a carrier, such as rosin or resin that is not evaporated, acid or salt deposits, and the removed oxides. If not removed, this residue can be detrimental to the long-term reliability of an electronic package. The resin can also absorb water and become an ionic conductor, which could result in electrical shorting and corrosion. Additionally, the residual activator can, over a period of time, corrode the soldered components and cause electrical opens.
When a flux is used that leaves corrosive and/or hygroscopic residues, post-soldering cleaning using chlorinated fluorocarbons (CFCs), organic solvents, semi-aqueous solutions, or water is required. For this type of process, in addition to volatile organic compound emissions from the soldering process, the cleaning process results in emission of CFCs and waste water. These emissions detrimentally add to environmental pollution and production costs.
To solve the problem of cleaning after reflow welding, no-clean fluxes have been introduced into the reflow soldering industry. Instead of the residue remaining on the circuit board after reflow welding, these no-clean fluxes are designed to undergo chemical decomposition at a given temperature, also known as pyrolyzation, during which the residue becomes a vapor and is emitted into the furnace atmosphere. Because this flux leaves little or no residue, the need to clean the circuit board after reflow welding is negated.
A problem associated with the use of no-clean fluxes, however, is that the residue vapor emitted into the furnace atmosphere via pyrolyzation tends to condense very quickly onto cool surfaces within the reflow furnace. This problem, although very prevalent with no-clean fluxes, also exists with fluxes that require cleaning after reflow. The condensed residue can cause many problems within the reflow furnace. For example, the condensed residue is often a liquid that can build up within the furnace and drip back onto the circuit board.
Another problem associated with the residue vapor is that the residue tends to build up on surfaces within the furnace such as fan blades, air amplifiers, feed lines, motor shafts, or heat exchangers. With heat exchangers, the residue will impair the efficiency of the heat exchanger and eventually require the heat exchanger to be cleaned. The cleaning of the heat exchanger and other components in the reflow furnace requires that the operation of the reflow furnace be halted. Also, in many cases the heat exchanger and other components must be removed from the reflow furnace and placed in a solvent bath. It can be recognized that for safety purposes, the furnace must be in a cooled state before the components of the furnace can be cleaned. The downtime of the reflow furnace associated with this periodic cleaning adds to the cost of production.
Many different type of systems have been employed to minimize problems associated with the condensation of residue within the reflow furnace. In some systems, the furnace atmosphere is exhausted to remove the residue vapor. In other systems, the furnace atmosphere is recycled through filters that remove the residue vapor. However, with each of these systems, the residue vapor can still condense upon surfaces within the reflow furnace, and therefore will still require cleaning of the furnace. Accordingly, a need exists for an improved method of operating a reflow furnace that reduces or eliminates the requirement to clean condensed residue within the reflow furnace.
This and other needs are met by embodiments of the present invention which provide a method of cleaning residue on surfaces in a reflow furnace. The method comprises introducing a solvent into the furnace chamber; reacting the solvent with the residue to form a product; and, removing the product from the furnace chamber.
By reacting the residue to form a product and then removing the product from the furnace chamber, the present invention removes residue formed on surfaces within the reflow furnace, thereby avoiding the need to shut down the furnace and clean the surfaces within the furnace. Also, having clean surfaces within the reflow furnace allows for greater efficiency of heat exchangers within the furnace and leads to less contamination of semiconductor devices being reflowed within the reflow furnace.
A further aspect of the present invention is the introduction of a gaseous solvent into the furnace chamber. The solvent can be an etch gas, such as oxygen, carbon monoxide, nitrous oxide, and water vapor. A gaseous solvent is better able to access all potentially contaminated surfaces within the reflow oven. Furthermore, the product of the solvent can also be gaseous. This advantageously allows the product to be exhausted from the reflow furnace.
Another aspect of the present invention is that the solvent used during certain embodiments of this process reacts with the residue at a temperature within the operating range of the reflow furnace. Advantageously, this allows the etch gas to be introduced into the process during the operation of the reflow furnace without having to make any changes in temperature to the reflow furnace. The operating temperature of the furnace during reflow is from about 220xc2x0 C. to about 280xc2x0 C. in certain embodiments.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.